Retrieving Default Values For Parameters In A Stored Procedure.

I'm generating a list of parameters needed by stored procedures, and
I'd like to know which ones have default values assigned to them.
To retrieve the parameter information I use:



sp_sproc_columns @Procedure_Name='InsertUser''



However, the column that is supposed to give the default value,
'COLUMN_DEF' always returns as NULL, even when that column has a
default value assigned to it.
i.e.

CREATE PROCEDURE InsertUser
@UserID INT = 10,
.....



And then if I do a sp_sproc_columns @Procedure_Name='InsertUser'', the COLUMN_DEF value for the @UserID column is still NULL.

Does anyone know what I'm doing wrong and how I can retrieve the default value?

Thanks